The present invention relates to power supply equipment for motor vehicles, including a motor generator driven by an inverter, and more particularly to power supply equipment for a motor vehicle, suitable for increasing the life of a battery, using an electrical double layer capacitor.
Recently, a system has been developed, which has a single motor generator that is used both as a conventional starter and a conventional alternator in which 42 volts are employed as a power supply voltage in order to improve a fuel efficiency in the vehicle. The motor generator is ordinarily driven by an inverter, and has an idle stopping function that stops the engine when the vehicle stops and that re-starts the engine when the vehicle is driven, a regenerative braking function that charges the battery with decelerating energy by a generator operation performed when the vehicle is decelerated, and an assisting function that applies a drive torque to a driving shaft in acceleration.
Application of a lead acid battery chargeable/dischargeable with a large electrical current at low cost has been studied as a battery for power supply. Since the output of this battery can lower at low temperatures/low charge rates, the engine is difficult to start up. Thus, it is necessary for the battery to have a sufficient capacity. Charging/discharging the battery with a large current affects the life of the battery adversely, and it is necessary to restrict the charging current especially.
In view of these problems, power supply equipment for motor vehicles has been studied that is intended to increase the service life of the battery by using a capacitor of an electrical double layer having a long life compared to the battery and being chargeable/dischargeable with a large current.
A first example is JP-A-10-184506 and 10-191576, which disclose power supply equipment for a motor vehicle, comprising a capacitor of an electrical double layer and a battery connected through a diode, a relay switch and a resistor in order to start up the engine with power stored in the capacitor.
A second example is JP-A-4-271209 that discloses power supply equipment for a motor vehicle, in which a capacitor of an electrical double layer and a battery are connected in parallel and also connected to corresponding switches in series in order to adjust quantities of energy stored in the capacitor and the battery, respectively, by controlling the operations of the switches.
A third example is JP-A-2000-156919 that discloses power supply equipment for a motor vehicle, in which an inverter is connected at its output to a capacitor of an electrical double layer, which is then connected to a battery through a DC/DC converter.
In the first example, however, since it is only possible to either charge the capacitor or the battery during the charging operation, the capacitor cannot increase the battery life by absorbing large transitional current.
In the second example, the capacitor cannot absorb a ripple current due to the switching of the inverter when the switch of the capacitor is off, and a capacity of an filter capacitor of the inverter cannot be reduced. Thus, it is impossible to achieve sufficient cost reduction.
In the third example, an increase in the cost cannot be avoided due to the employment of the DC/DC converter.